Anti-backout arthroscopic uni-compartmental prosthesis

ABSTRACT

An improved uni-compartmental implant has a shaft having a proximal end attached to a head and a distal end, and one or more raised portions spaced apart along the shaft to resist back-out. The length between the head and distal end is preferably less than 50 mm, the distal end of the shaft has a diameter on the order of 2 to 3 mm, the proximal end of the shaft has a diameter on the order of 2 to 4 mm, and the head has a diameter ranging from 4 mm or less to 20 mm or more, making the device suitable for knee arthroscopy and other applications. The shaft and/or raised portions may include a bone-ingrowth or bone-ongrowth surface, and the shaft and/or raised portions may be made of a fiber-metal. The head portion is preferably ceramic, though a chrome-cobalt alloy, titanium, or other bio-compatible material may be used. The head portion may have a bi-convex shape, a plano-convex shape, or a concave-convex shape.

FIELD OF THE INVENTION

This invention relates generally to joint-related prosthetic devicesand, in particular, to an arthroscopic, uni-compartmental prosthesis.

BACKGROUND OF THE INVENTION

Due in part to an aging population that wishes to remain active,arthritis of the knee is approaching epidemic proportions in the U.S.Another factor is obesity, since the knees bear much of increased weightin the body. It is estimated that approximately 750,000 surgicalprocedures are done in the U.S each year for knee problems, includingtotal-knee replacements, partial-knee replacements, and arthroscopicprocedures.

Quite often, patients treated with knee arthroscopy for arthritis of theknee do very poorly. There are a number reasons for this, but the lowrate of success is largely due to the fact that these patients have asmall area of their cartilage which is denuded of cartilage and theycontinue to have pain. Although the area of cartilage eburnation is notlarge enough to warrant joint replacement procedure, it is large enoughto cause continued problems and significant patient dissatisfaction.

Uni-compartmental knee procedures have therefore become more popular inrecent years. One reason is that smaller incisions are now used, to theextent that uni-compartmental knees are now carried out through aso-called minimally invasive approach. Still, however, in many case thisinvolves a 4-inch incision, significant soft tissue dissection, andsignificant morbidity for the patient.

To improve these procedures, various implants and techniques are beingdevised. One of many is disclosed in Published U.S. Patent Application2002/0099446 A1. This reference discloses a knee-joint prosthesiscomprising at least one femoral component and at least one tibialcomponent. The femoral component includes a first portion adapted forfixable attachment to a distal end of a femur and a second portionformed with a bearing surface. The femoral component is sized so as topermit attachment to the femur of a patient without severing at leastone the cruciate ligaments. The tibial component has a first surfacethat is adapted to cooperate with a patient's tibia, while a secondsurface of the tibial component is adapted to cooperate with the femoralcomponent. The tibial component is sized so as to permit attachment tothe patient's tibia without severing at least one of the cruciateligaments.

Despite advances such as these, however, the need remains for animproved implant, preferably one that resists back-out.

SUMMARY OF THE INVENTION

This invention resides in an improved uni-compartmental implantincluding a shaft having a proximal end attached to a head and a distalend, and one or more raised portions spaced apart along the shaft toresist back-out. The length between the head and distal end ispreferably less than 50 mm, the distal end of the shaft has a diameteron the order of 2 to 3 mm, the proximal end of the shaft has a diameteron the order of 2 to 4 mm, and the head has a diameter ranging from 4 mmor less to 20 mm or more, making the device suitable for kneearthroscopy and other applications.

The shaft and/or raised portions may include a bone-ingrowth orbone-ongrowth surface, and the shaft and/or raised portions may be madeof a fiber-metal. The head portion is preferably ceramic, though achrome-cobalt alloy, titanium, or other bio-compatible material may beused. The head portion may have a bi-convex shape, a plano-convex shape,or a concave-convex shape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing that illustrates a preferred embodiment of theinvention.

FIG. 2 is a drawing that illustrates an alternative head design;

FIG. 3 is a drawing that illustrates a different alternative headdesign; and

FIG. 4 is a drawing that illustrates yet a further alternative headdesign.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a preferred embodiment of the invention. Theimplantable device, shown generally at 100, includes a shaft portion 102having a head portion 104 and one or more raised portions 106 to resistpull-out. In terms of dimensions, the length of the device is preferablyon the order of 25 mm, though length in excess of this, or on the orderof 10 mm or less, may be more appropriate depending upon theapplication. The distance between the raised portions, “D” is preferablya few millimeters; for example, between 5 and 10 millimeters, dependingupon the number used and other considerations.

The shaft portion 102 preferably tapers from a diameter at “B” of 3 mm,or less, to a diameter at “A” of 2.5 mm, or thereabouts. The head 104will preferably be offered in different diameters, such as 4, 6, 8, 10,15, and 20 mm, and so forth, in which case smaller-diameter heads mayhave smaller dimensions of A and B, and larger-diameter heads may havelarger dimensions of A and B. Smaller dimensions may use less raisedportions 106, whereas larger dimensions may use more of them.

Although the head portion 104 is generally shown as a bi-convex shape,other head geometries may be appropriate, such as plano-convex,concave-convex, and different radii of curvature, whether concave orconvex surfaces are used. In addition, although the edge of thebi-convex surfaces of the head 104 are shown in the drawing as smoothlytransitioning through a smaller radius, the sharp edge may alternativelybe used. FIGS. 2-4 show three possible alternative head configurations.

In terms of materials, the head portion may be made of any appropriatebio-compatible material, such as chrome cobalt or titanium, though inthe preferred embodiment, ceramic is used. The shaft 102 and raisedportions 106 preferably include some type of porous ingrowth or ongrowthsurface such as hydroxyapetitite, and such surfaces may be used inconjunction with raised bumps to further assist in preventing backout.Although a metallic shaft in raised portions may be used, whenavailable, a fibermetal one is the preferred technology.

The inventor has also devised a way to perform a procedurearthroscopically without large incisions so that we could take care ofthese patchy areas of ebumated bone within an isolated condyle in theknee. The procedure could be done on the lateral or medial side, and ifthe technique was altered slightly, it could even be applied to thepatellofemoral groove. The technique would involve a variation of aprocedure known as the OATS procedure. In this procedure, osteoarticulartransfer of tissue is performed by using essentially a trephine to coreout a plug of bad bone where the cartilage has been worn away oreburnated and then an area of the knee is harvested that has articularcartilage covering it but is not needed, for instance, the inner portionof the patellofemoral groove or inner portion of the medial and lateralfemoral condyle along the intercondylar notch. These tissue plugs, whichcontain bone and cartilage, are then transferred over to this area. Thisprocedure has had moderate success. It is mostly used for young peoplewho have isolated articular defects.

According to this invention, the OATS procedure is converted to anarthroplasty technique where, instead of a plug of bone and cartilage,the plug of FIG. 1 is instead used. The area of defect would beisolated, identified, and measured, then a guide wire would be placedcentrally into the defect. Over the guide wire, a cannulated reamerwould be placed that we would ream to a specified depth. This wouldestablish the canal size for the ‘stem’ of the prosthesis. We would thenover-ream with a secondary reamer which would then establish the sizefor the ‘rounded head’ of the prosthesis. At this point, the prosthesiscould be either press fitted or cemented into place in the defect.

I believe that this technique would have significant advantages over theOATS procedure since this would be more rigidly fixed and it would besealing the defect with cement and/or cobalt chrome. It would be moreapplicable for the elderly population as they have more of a geographicebumation of bone as opposed to small circumscribed lesions that areapplicable to the OATS procedure. I would envision that for a typicalarthritic knee, one would need multiple plugs of cobalt chrome thatcould be placed in these areas. With relative ease, the surgeon couldplace as many as four or five of these circular plugs in the knee totake care of the eburnated areas where the bone is exposed. An inventorywould be maintained that would come in different diameter sizes and stemlengths for the prostheses. They could easily be used in a right or aleft knee and each prosthesis implanted would be a separate charge. Theyare relatively small; therefore, they would not occupy a large amount ofshelf space at the hospital or in the local distributor's office. Theinstrumentation would be easy to design and would fit very nicely in aself-contained unit.

In rare situations, we would find eburnated bone on the tibial side.This would obviously be more difficult to reach because of the anatomyof the knee. However, it is conceivable that lesions within the anteriortwo-thirds of the knee on the tibial plateau could easily be re-surfacedin a manner such as I just described. These plugs will actually be moreflat as opposed to a slightly rounded plug that would be used on thefemoral side.

1. A uni-compartmental implant, comprising: a shaft having a proximalend attached to a head and a distal end; and one or more raised portionsspaced apart along the shaft to resist back-out.
 2. Theuni-compartmental implant of claim 1, wherein the length between thehead and distal end is less than 50 mm.
 3. The uni-compartmental implantof claim 1, wherein the distal end of the shaft has a diameter on theorder of 2 to 3 mm.
 4. The uni-compartmental implant of claim 1, whereinthe proximal end of the shaft has a diameter on the order of 2 to 4 mm.5. The uni-compartmental implant of claim 1, wherein the shaft includesa bone-ingrowth or bone-ongrowth surface.
 6. The uni-compartmentalimplant of claim 1, wherein the raised portions include a bone-ingrowthor bone-ongrowth surface.
 7. The uni-compartmental implant of claim 1,wherein the shaft is made of a fiber-metal.
 8. The uni-compartmentalimplant of claim 1, wherein the raised portions are made of afiber-metal.
 9. The uni-compartmental implant of claim 1, wherein thehead portion is ceramic.
 10. The uni-compartmental implant of claim 1,wherein the head portion is constructed of a chrome-cobalt alloy orother bio-compatible alloy.
 11. The uni-compartmental implant of claim1, wherein the head portion has a bi-convex shape.
 12. Theuni-compartmental implant of claim 1, wherein the head portion has aplano-convex shape.
 13. The uni-compartmental implant of claim 1,wherein the head portion has a concave-convex shape.
 14. Theuni-compartmental implant of claim 1, wherein the head portion has adiameter ranging from 2 mm to 20 mm.